This PR adds lib.attrsets.recurse function (based on mapAttrsRecursiveCond) that performs graph traversal on nested attribute sets and allows converting other functions like mapAttrs, concatMapAttrs, mapAttrsToList, etc, to recursive versions.

It also updates mapAttrsRecursiveCond and filterAttrsRecursive implementations to use recurse. In particular, this allows reusing filterAttrs for recursive implementation and should benefit from #345547.

See also #237776

Supersedes #221608 (lib.{collect',flattenAttrs} can be implemented using attrsets.recurse). E.g.

lib.attrsets.recurse lib.concatMapAttrs
  (_: v: v != { })
  (p: v: { ${lib.concatStringsSep "/" p} = v; })
  { a = 1; b.c.d = 2; x.y = { }; }
⇒ { a = 1; "b/c/d" = 2; "x/y" = { }; }

concatMapAttrsToList = f: attrs: builtins.concatMap (name: f name attrs.${name}) (builtins.attrNames attrs)
lib.attrsets.recurse concatMapAttrsToList
  (_: v: v != { })
  (path: value: [{ inherit path value; }])
  { a = 1; b.c.d = 2; x.y = { }; }
⇒ [
    { path = [ "a" ]; value = 1; }
    { path = [ "b" "c" "d" ]; value = 2; }
    { path = [ "x" "y" ]; value = { }; }
  ]

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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